There are many situations in which downhole tools must be selectively actuated. However, communicating with the tools to cause actuation can be difficult in the downhole environment. Systems such as RFID systems exist but these are complex, expensive and prone to failure. Indeed, any form of electrical, electronic or magnetic device is often not robust enough to withstand the harsh downhole environment.
During hydraulic fracturing of a multiple zone well, a series of tools, or clusters of tools, are provided at each zone, and each downhole tool needs to be actuated and fluid is diverted to flow outwards to fracture the well. The actuation must be performed in a sequential manner to allow the borehole to be progressively fractured along the length of the bore, without leaking fracture fluid out through previously fractured regions.
Due to the expense and frequent failure of electronic or electrical devices, the most common approach to tool actuation is still fully mechanical. Balls of ever increasing size are dropped down a tubular positioned within the well bore. The tools are configured so that the first dropped ball, which has the smallest diameter, passes though the first and intermediate tools, which have a ball seat (hereinafter referred to as a valve seat) larger than the ball, until it reaches the furthest away tool in the well. This furthest away tool is configured to have a valve seat smaller than the first dropped ball so that the ball seats at the tool to block the main passage and cause transverse ports to open thus diverting the fluid flow. Subsequently dropped balls are of increasing size so that they too pass through the nearest tools but seat at further away tools which have a suitably sized valve seat. This is continued until all the tools have been actuated in the order of furthest away to nearest.
Therefore, this approach does not involve counting the dropped balls. Balls which are too small for a particular tool are simply not registered. However, this approach has a number of disadvantages. The number of tools with varying valve seats that can be used is limited in practice because there must be a significant difference in the size of the seat (and therefore the ball) so that the ball does not inadvertently actuate previous tools. Also, the valve seats act as restrictions to flow through the tubular which are always undesirable. The smaller the seat the greater the restriction.
It is desirable to provide an apparatus which allows: actuation of a large number of downhole tools; and/or downhole tools with the same size of valve seat; and/or valve seats with the largest possible diameter.